A known gear machining apparatus, which machines an internal spur gear by skiving, is disclosed in the following reference: KOJIMA Masakazu, and NISHIJIMA Komio. “Gear Skiving of Involute Internal Spur Gear: Part 1, On the Tooth Profile” Transactions of the Japan Society of Mechanical Engineers 39(324), 2580-2586, (hereinafter referred to as non-patent reference 1). The non-patent reference 1 discloses that a tooth profile is defined by defining factors, for example, a rake angle of specifications for a cutter, a cutting angle, a rake face re-cutting amount, and a tooth number ratio.
A skiving method for attaining high cutting efficiency when machining an internal spur gear, and modifications of tooth profile of a cutter is disclosed in the following reference: NISHIJIMA Komio, KOJIMA Masakazu, and YAMADA Toyo “Gear Skiving of Involute Internal Spur Gear: Part 2, On the Practical Problems” Transactions of the Japan Society of Mechanical Engineers 40(329), 260-268, (hereinafter referred to as non-patent reference 2).
JPH07-84896B (hereinafter refereed to as patent reference 1) discloses a flexible meshing type gear device as a gear device applied with the internal gear. The flexible meshing type gear device includes a rigid circular internal gear, a flexible external gear arranged inside the internal gear, and a wave generator flexibly deforming and rotating the external gear in the deformed configuration.
The gear device disclosed in the patent reference 1 is named as a strain wave gearing device in which the number of teeth of the external gear is defined to be less than the number of teeth of the internal gear, and only two teeth corresponding to positions of portions of an elliptic longitudinal shaft of a cam plate of the external gear engage with the teeth of the internal gear by the deformation of the external gear by the cam plate. By rotating the cam plate with the foregoing constructions, engaging points are moved while elastically deforming the external gear in response to the movement of the elliptic longitudinal shaft to obtain a greater reduction ratio on the basis of a difference of the teeth numbers between the internal gear and the external gear.
As described in the patent reference 1, because the strain wave gearing device is configured to move a region to flex in response to the rotation of the cam plate by engaging a portion of the external gear to the internal gear by flexing the external gear by the cam plate, the external gear and the internal gear are required to be formed with high precision.
Regarding the internal gear, a gear shaper may be applied to machine the internal gear with high precision. However, although the machining is achieved with high precision with the gear shaper, the machining time with the gear shaper is long thus increasing manufacturing costs of the strain wave gearing device.
Alternatively, a skiving may be applied for machining the internal gear. The skiving is favorable for reducing the machining time of the internal gear because of high cutting efficiency. Notwithstanding, it has been considered that the machining with high precision is unlikely to be obtained by skiving.
Reasons that the machining with high precision is unlikely obtained by the skiving are that many parameters (factors) are required to be defined when machining, degrees of variations of machining precision (tooth profile error) are great in accordance with the dispersions of the parameters, and the tooth profile error is sensitively reflected onto the machining precisions. As described in the non-patent reference 1 and the non-patent reference 2, for example, a crossed axes angle of an axis of an object to be machined (a work) and an axis of the cutter, a distance between the axes, and a value of radius of a base circuit of the cutter, are included as the parameters defined when machining. By setting the multiple parameters appropriately, the high machining precisions can be obtained.
According to the skiving, dispersions of the machining precisions are caused by variations of tooth profile of a machined work gear by, for example, errors in cutter production, setting errors of machining machine, and when putting an edge on the cutter, which are necessary for normal production activities. The references disclose that the errors in the tooth profile cause a tendency similar to the case where the radius of the base circle of the cutter is varied.
A need thus exists for a gear machining apparatus, a cutter and a strain wave gearing device which is not susceptible to the drawback mentioned above.